When you buy coffee on Algrano in your local currency (like GBP) but the producer is paid in USD, the price you see already includes an exchange rate. This rate might differ slightly — usually by 2–3% — from what you see online.
When you search “USD to GBP” on Google, you see the mid-market rate — a midpoint between what banks buy and sell currency for.
No one actually trades at that number. Real exchange rates always include a spread, which covers transaction costs and market risks — just like the “buy” and “sell” rates you see at the Post Office.